The present invention pertains to a method for manufacturing a tube and a sheet of niobium-containing zirconium alloys for high burn-up nuclear fuel, comprising melting a metal mixture comprising of zirconium and alloying elements to obtain an ingot, forging the ingot at xcex2 phase range, xcex2-quenching the forged ingot in water after a solution heat-treatment at 1015-1075xc2x0 C., hot-working the quenched ingot at 600-650xc2x0 C., cold-working the hot-worked ingot in three to five times with intermediate vacuum annealing, and final vacuum annealing the cold-worked ingot at 440-600xc2x0 C.
In the development of nuclear reactors, such as pressurized water reactors (PWR) and boiling water reactors (BWR), zirconium alloys have been widely used in nuclear reactor applications, including nuclear fuel cladding, nuclear fuel assembly components, and reactor core components.
Among zirconium alloys developed up to now, Zircaloy-2 (Sn: 1.20-1.70%, Fe: 0.07-0.20%, Cr: 0.05-1.15%, Ni: 0.03-0.08%, O: 900-1500 ppm, Zr: the balance) and Zircaloy-4 (Sn: 1.20-1.70%, Fe: 0.18-0.24%, Cr: 0.07-1.13%, O: 900-1500 ppm, Ni: less than 0.007%, Zr: the balance), which include Sn, Fe, Cr and Ni, have been widely used. (Herein, % means % by weight).
Recently, the high burn-up/extended cycle nuclear fuels have been used to improve economic efficiency of nuclear reactors. In the case of the conventional Zircaloy-2 and Zircaloy-4, many problems are caused in terms of corrosion and mechanical properties. Hence, Nb known to be used in improving mechanical strength and creep resistance, as well as improving corrosion resistance of zirconium alloy and low hydrating, is added to zirconium alloys used for fuel cladding and space grids of the high burn-up/extended cycle nuclear fuel.
An important factor affecting corrosion and mechanical properties in zirconium alloys is the chemical composition of the alloy and also its amount. However, corrosion and mechanical properties of zirconium alloys having the same composition are greatly changed depending on annealing conditions and working degree.
In particular, physical properties of Nb-containing zirconium alloys depend on the manufacturing processes, so that the optimal manufacturing processes should be established.
In the prior arts related to manufacturing processes of Nb-containing zirconium alloys useful as cladding rods of high burn-up/extended cycle nuclear fuels, U.S. Pat. No. 5,648,995 refers to a method for manufacturing cladding tubes made out of zirconium alloys containing Nb: 0.8-1.3 wt %, Fe: 50-250 ppm, O: 1600 ppm or less, C: 200 ppm or less, Si: 120 ppm or less. In this patent, an ingot of Nb-containing zirconium alloy is heated to between 1000xc2x0 C. and 1200xc2x0 C., xcex2-quenched in water, heated to the range of 600xc2x0 C. to 800xc2x0 C. and then extruded. Thereafter, cold rolling is conducted in four to five passes with intermediate heat treatments in the range 565xc2x0 C. to 605xc2x0 C. for 2-4 hours and a final heat treatment is performed at 580xc2x0 C., thereby manufacturing cladding tubes of nuclear fuels. As such, in order to improve creep resistance, Fe in the alloy composition is limited to an amount of 250 ppm or less, and O is limited to the range of 1000-1600 ppm.
U.S. Pat. No. 5,940,464 discloses a method of manufacturing alloys comprising Nb: 0.8-1.8 wt %, Sn: 0.2-0.6 wt %, Fe: 0.02-0.4 wt %, C: 30-180 ppm, Si: 10-120 ppm, O: 600-800 ppm and Zr the balance. The bars of alloys are heated at 1000-1200xc2x0 C. and then quenched. The melted bar is drawn into a blank after heating to a temperature in the range of 600xc2x0 C. to 800xc2x0 C., followed by annealing the drawn blank at a temperature in the range 590xc2x0 C. to 650xc2x0 C. The annealed blank is cold-worked in at least four passes into a tube, with intermediate heat treatments at temperatures in the range 560xc2x0 C. to 620xc2x0 C. Next, a final heat treatment step for recrystallization at a temperature in the range 560xc2x0 C. to 620xc2x0 C., and a final heat treatment step for stress relief at 470-500xc2x0 C., are carried out.
U.S. Pat. No. 5,838,753 refers to a process for fabricating a nuclear fuel cladding tube, comprising xcex2-quenching a zirconium alloy billet comprising of Nb 0.5-3.25% and Sn 0.3-1.8% by heating to a temperature in the xcex2range above 950xc2x0 C. and rapidly quenching the billet below a transformation temperature from xcex1+xcex2 to a to form a martens tic structure, extruding the billet at below 600xc2x0 C., forming a hollow, annealing the said hollow by heating at a temperature up to 590xc2x0 C., pilgering the said annealed hollow, and finally annealing at a temperature up to 590xc2x0 C. to form the said cladding tube for nuclear fuel. This patent also comprises of the alloy having a microstructure of second phase precipitates of xcex2-niobium distributed uniformly, intragranularly and intergranularly forming radiation resistant second phase precipitates in the alloy matrix so as to increase the resistance to aqueous corrosion, compared to that of Zircaloy, when irradiated to high fluency. The xcex2-quenching step is conducted below 250xc2x0 C. at a cooling rate greater than about 300 K/sec, and the second phase precipitates in the alloy have an average diameter of 80 nm. In the alloy further comprising Si: 150 ppm or less, C: 50-200 ppm and 0: 400-1000 ppm, the second phase precipitates have an average diameter of 60 nm.
EP 0 198 570 B1 relates to a process for fabricating a thin-wall tube (less than 1 mm in thickness) from 1.0-2.5 wt % of Nb-added zirconium alloy selectively containing Cu, Fe, Mo, Ni, W, V, and Cr as well as homogeneously and finely dispersed particles formed by xcex2-treating a niobium-containing zirconium alloy billet; extruding the said xcex2-treated billet at a temperature less than 650xc2x0 C. to form a tube shell; further deforming the said tube shell by cold working the same in a multiple of cold working stages; annealing the said tube shell, between the stages of the cold working, at a temperature below 650xc2x0 C.; and finally annealing the resultant tube at a temperature below 600xc2x0 C., so as to control a microstructure of the material having the niobium-containing particles of a size below about 80 nm homogeneously dispersed therein. The 1-2.5 wt % Nb-added alloys are extruded, annealed at 500-600xc2x0 C., preferably 524xc2x0 C. for 7.5 hours, and then finally annealed at 500xc2x0 C., preferably 427xc2x0 C. for 4 hours. The tube shell after extrusion is xcex2-annealed at 850-1050xc2x0 C. and then quenched.
Additionally, U.S. Pat. No. 5,230,758 discloses that zirconium alloy comprising Nb: 0.5-2.0 wt %, Sn: 0.7-1.5 wt %, Fe: 0.07-0.14 wt %, Cr: 0.025-0.08 wt %, Crxe2x80x94Ni 321 ppm or less, 0.03-0.14 wt % of at least one of Cr and Ni, at least 0.12 wt % total of Fe, Cr and Ni, C: 220 ppm or less, is subjected to a post extrusion annealing and a series of fabrication step. Intermediate annealing temperature is 645-704xc2x0 C. and the alloy is subjected to xcex2-quenching two steps prior to a final sizing.
Therefore, under study has been the method of making Nb-containing zirconium alloys for high burn up/extended cycle nuclear fuel with improving the corrosion resistance and strength by changing the kind and amount of added elements and conditions of working and annealing.
Leading to the present invention, the intensive and thorough research on a novel method for manufacturing Nb-containing zirconium alloys with excellent corrosion resistance and mechanical properties, carried out by the present inventors aiming to avoid the problems encountered in the prior arts, resulted in the finding that added elements are changed in kinds and amounts, and also cold working is conducted 3-5 times, annealing being performed at relatively low temperatures, and average size and annealing conditions of precipitates in the alloy matrix are quantitatively determined by use of the accumulated annealing parameter (xcexa3 A), thereby developing an optimized method for manufacturing zirconium alloys comprising 0.05-1.8% of niobium and Sn, Fe, Cr, Mn, and Cu for nuclear fuel cladding tube.
Therefore, it is an object of the present invention to provide a method for manufacturing a tube and a sheet of Nb-containing zirconium alloys with excellent corrosion resistance and mechanical properties, useful as high burn up/extended cycle nuclear fuel.